Microfiltration, ultrafiltration, and nanofiltration of water by membrane modules and other technologies have been employed in various applications where high purity water is required.
There are different degrees of filtration, as well as different types of filter technologies, which are selected and operated in accordance with the material to be separated and the quality required. For example, the microfiltration membrane filter modules can efficiently remove fine particles and microorganisms measuring 10 μm or smaller, particularly 1 μm or smaller, and are therefore commonly used in the purification of potable water supplies and waste water.
One common type of microfiltration module uses a hollow fiber membrane. These hollow fibers can be bound in a cylindrical tube bundle type configuration to maximize filtration surface area while minimizing module volume. An alternative approach is to start with a membrane sheet, fold pleats into the sheet and then arrange the pleated sheet in either a cylindrical configuration or a planar configuration.
Among these approaches, the hollow fiber membrane module enables a large membrane filtration surface area per unit volume and is often preferably used. The hollow fiber-based microfiltration membrane module is routinely used in filtration to remove suspended solids and most bacteria and viruses to produce clear filtered water.
The hollow fiber membrane module is also used in large-scale purification plants having processing capacity over 10,000 m3 per day, in the case of potable water purification. In such an application, a large number of hollow fiber membrane modules are used to secure a large membrane area.
After a period of continuous filtering operation, however, the micropores of the membrane become clogged with debris, resulting in a decrease in flow rate of the filtered water and an increase in the filtration back pressure, thus making it necessary to either perform a major cleaning of the membrane modules or replace them. Frequent major cleaning and especially frequent change of the membrane module is economically disadvantageous.
In order to minimize major cleaning or replacement of filters, less expensive operations to partially restore the filtering function are typically carried out. In the case of an external pressure-driven hollow fiber membrane module, for example, the following processes have been used to partially restore filter function: 1) Backwashing, wherein filtered water is passed in a reverse direction from the inside to the outside of the hollow fiber membrane to dislodge debris; 2) Scrubbing of the fibers' external surfaces, initiated by pneumatically agitating the fluid surrounding the fibers (the combination of air bubbles and fluid stripping debris away from the membrane surfaces); or 3) A combination of these cleaning operations.
While beneficial, these mechanical and backwashing methods are not completely effective in removing all contaminant material and over time their efficacy gradually decreases as the membranes become fouled by material which is not so readily removed by these means. Because of the nature of the material being filtered, which is often surface water, ground water or material passing through membrane bioreactors and the like, the fouling agents are generally biological/organic in nature and also usually contain foulants of an inorganic nature.
Major cleanings typically employ chemical cleaning agents to remove foulants from membrane pores and surfaces. Because of the presence of more than one type of foulant (bio/organic foulants on the one hand, and inorganic foulants on the other), a dual chemical clean is usually required to restore the membrane's performance. An oxidant or caustic agent is used to remove organic foulants, and acids or chelating agents are used to remove inorganic materials fouling the membrane. The two chemical cleanings are carried out in series. This major cleaning process can be done in place, or the filters can be removed for cleaning. In either a clean-in-place or a remove-to-clean operation, the downtime is normally from four hours to two days. For many applications, such lengthy downtimes are cost prohibitive.
One approach to clean membrane modules is to use large quantities of chlorine in a cleaning process performed after the membranes became clogged. This “clean in place” process could take days to complete, was somewhat manual and required disassembly. In addition, it was found to cause significant chlorine dosing. Because chlorinated organic by-products are carcinogenic, the procedure could not be used in a potable water treatment system.
To avoid the potential problems associated with chlorine, this first approach instead proposed to use a monopersulfate cleaning solution. The monopersulfate cleaning solution could be fed into the feed side of hollow fiber membranes, so that membranes could stand and soak in the solution for a desired period, for example, several hours. It was also proposed to inject the monopersulfate cleaning solution to the filtrate side during a backwash mode, or to use the monopersulfate cleaning solution in repeated cycles of backwash and soaking by recirculating the persulfate solution through the membrane or membrane system. In a continuous process, the monopersulfate cleaning solution concentration could be injected immediately upstream of the membrane.
A second approach is to periodically perform a cleaning operation after a pre-determined period of filtration. According to this proposal, the cleaning operation may include scrubbing cleaning performed using air supplied by a blower and backwashing by reversing the flow of filtrate from a backwash tank. During backwashing, sodium hypochlorite would be injected into the backwash water from a chemical tank. With the second approach, scrubbing cleaning or backwashing could be performed alone or scrubbing cleaning and backwashing could be combined. For example, a membrane filtration device could be operated in the sequence of (i) filtration (ii) scrubbing cleaning, (iii) filtration, (iv) backwashing and so forth.
The primary drawback of most backwash operations is that they require significant amounts of filtered product water to be effective and this reduces the overall output of the filtering process. This production penalty becomes even more significant when chemical additives are used in the process since flushes have to be incorporated to ensure the additives do not enter the flow stream.